Vasopressin is a neuropeptide which is produced in the posterior pituitary gland and is known to be able to cause vasoconstriction. It is not known, however, whether this agent or any vasoconstrictor can actually produce sufficient coronary artery vasoconstriction to overcome the system of autoregulation and produce myocardial ischemia. Thus, in a group of dogs we infused vasopresin directly into the coronary artery and found a reduction in coronary blood flow of 40-50%. This reduction in coronary blood flow is associated with abnormal contraction patterns of the heart measured by nuclear medicine gamma camera computer system. In addition, a fiber optic probe, which measured intramyocardial pH revealed that this decrease in coronary blood flow was indeed functionally significant because it caused development of myocardial acidosis with a drop in pH. Thus, the present studies are important because they show that vasopresin, as an example of a endogenous product which can cause vasoconstriction, is capable of reducing coronary blood flow sufficient to overcome the system of autoregulation and actually produce myocardial ischemia.